igig] BURGER— CUNN INCH AMELIA 135 



the environments were, in themselves, sufficient to induce zygo- 

 spore formation. 



The early mycologists believed that all the genera were alike 

 in their method of zygospore formation, but that their irregularity 

 of occurrence was to be accounted for by one factor or a com- 

 bination of the factors mentioned. Blakeslee, however, in 1904, 

 showed that the Mucorineae could be divided into 2 classes, 

 namely, homothallic and heterothallic forms, the homothallic 

 forms being those whose zygospores are formed by the conjugation 

 of gametes which are produced by hyphae of the same individual 

 mycehum. To this group belong such genera as Dicranophora , 

 Sporodinia, Spinellus, Zygorynchus, and some species of the genus 

 Mucor. The heterothaUic forms are those in which 2 kinds of 

 individual mycelia occur. If the 2 individual strains of a hetero- 

 thalHc form are grown on the same medium, zygospores are pro- 

 duced at the point where the hyphae of the 2 strains meet. In 

 this group belong most of the genera of the ]\Iucorineae. Those 

 2 strains Blakeslee first called plus and minus, but in later 

 papers he states that the sexes are as distinct as in higher organ- 

 isms; the plus he calls female and the minus male. In the Bio- 

 logical Bulletin for August 191 5 (p. 87) he says: "Conjugation in 

 the Mucors is as definitely a sexual process as the morphologically 

 more complex types of reproduction in higher forms, and the 

 sexes seem even more sharply distinct." In the homothallic form 

 Zygorynchus heterogamus one gamete is larger than the other; 

 Blakeslee calls the larger gamete female and the smaller male. 

 If the gametes are strictly male or female, as he maintains, there 

 must have been a segregation of the male and the female nuclei 

 in the respective gametes. In the heterothaUic forms the male 

 and female nuclei are segregated in different mycelia, and all the 

 gametes produced by a single mycelium, therefore, must be either 

 male or female. 



According to Blakeslee, whenever a plus strain meets a 

 minus strain of the same species, zygospores are produced. When 

 opposite strains of different species meet, progametes only are 

 formed, producing what he calls imperfect hybridization. No zygo- 

 spores are produced; not even are gametes formed. The stimulus 



